The present invention relates to the distribution of scrambled television programs over cable television networks or the like, and more particularly to a method and apparatus for preventing the absence of timing information on the sound carrier of a scrambled television signal from being used to detect the vertical interval for recovering horizontal synchronization information.
Techniques for scrambling the video portions of television signals are well known. For example, U.S. Pat. No. 3,813,482 to Blonder discloses a system for transmitting television signals wherein the video is scrambled by suppressing vertical or horizontal synchronization pulses to produce a shifting or rolling scrambled picture. In U.S. Pat. No. 4,542,407 to Cooper, et al, apparatus is disclosed for scrambling and descrambling television programs in which the horizontal synchronizing information is suppressed at a cable television headend, and then regenerated by a subscriber's cable television converter. U.S. Pat. Nos. 4,095,258 to Sperber, 4,163,252 to Mistry, et al, and 4,571,615 to Robbins, et al describe apparatus for decoding scrambled television signals.
The economic viability of subscription television programming, including cable television and satellite television services, is dependent on the ability of the transmitter to encode or scramble a television signal so that an unauthorized receiver will not be able to receive a viewable television picture. As indicated in the patents cited above, a common technique that has been employed to scramble video signals is to suppress the horizontal synchronization pulses below the average value of the video level. This causes the television receiver to unsuccessfully attempt to lock horizontally on random video peaks rather than on the horizontal synchronization pulses. The loss of effective horizontal synchronization also prevents the receiver from properly utilizing the color burst signal associated with the horizontal synchronization pulse, so that accurate color reproduction cannot be achieved.
In order for a receiver to be able to view the scrambled video signal, the suppressed synchronization pulses must be restored. Two techniques are commonly employed to allow the receiver to recover the suppressed sync and timing information. In one technique, a timing pulse is amplitude modulated on the FM audio carrier of the television signal, which is then detected in the audio portion of the receiver and used to generate the timing signals necessary to descramble the received video signal. This technique is the subject of U.S. Pat. No. 4,621,285 issued on Nov. 4, 1986 to Schilling et al and entitled "Protected Television Signal Distribution System", the entire disclosure of which is incorporated herein by reference.
In another known technique, some portion of the sync timing pulses, such as during the vertical blanking interval, is transmitted without suppression, i.e., "in the clear". The receiver phase locks to the clear or unsuppressed sync portion to create the required synchronizing and timing information for descrambling the video portions of the signal.
Neither of these known techniques are immune to unauthorized recovery by "pirate" descramblers. Such descramblers are known that will restore usable synchronization to virtually all known prior art sync suppressed video signals. One such device that can overcome the scrambling technique disclosed in the aforementioned Schilling et al patent relies on the absence of horizontal synchronizing pulse timing information on the sound carrier during the vertical blanking interval portion of the scrambled television signal. The absence of such information is detected, indicating the occurrence of the vertical blanking interval. The pirate descrambler uses this information to locate synchronization pulses on the video carrier during the vertical blanking interval. Such pulses can be detected because there is no video information to mask them during vertical blanking. The detected synchronization pulses are then used to phase lock an oscillator that provides synchronization pulses during the active video portions of the signal.
It would be advantageous to provide a system for scrambling the video portion of television signals that would preclude the detection of the vertical blanking interval from the absence of horizontal synchronizing timing pulses on the sound carrier. Such a system would defeat the ability of pirate descramblers to recover synchronization information during the vertical blanking interval. It would be further advantageous to provide such a system that will not interfere with the descrambling of scrambled signals by the millions of authorized descramblers already installed in the field. The present invention provides such a system.